Non-radiative recombination is one of the solar cells biggest enemies. Fortunately, unless radiative recombination, non-radiative recombination can be avoided and is therefore a key objective of our research group. Non-radiative recombination originates from traps and defect states in the bandgap allowing charges to relax to the ground state through the interaction with phonons and without the emission of photons. Unfortunately, the location of these defects and the dominant non-radiative recombination pathway is often unknown, which complicates a systematic optimization of the devices.
To quantify, and understand the complex recombination processes and mechanism in the multi-layered perovskite cells, the Perowskite Group applies various optical, and electrical measurement techniques in steady-state and in transient mode (Find out more).
Above, a solar cell is illustrated as a bucket that is constantly filled with water (which represents the generation current from the sun). The water level represents the open-circuit voltage (Voc) and the holes the various non-radiative recombination currents (i.e. recombining charges per time) that limit the cell. Note, under open-circuit conditions, the outflow from the bucket is always equal to the influx. However, the water level (i.e. the Voc) can be maximized by closing all holes for non-radiative recombination losses, only keeping the hole for radiative recombination. Our research on perovskite solar cells shows us that most charges recombine in the interface regions between the perovskite and the transport layers. The associated interfacial recombination current (Jrec) limits the achievable open-circuit voltage in the cells by ∆Voc=kT/q*ln(1/Jrec).